The present invention relates to digital communications systems and, more particularly, to demodulation of adjacent channel signals.
Today, digital communication systems are developing rapidly for both wireline and wireless applications. Wireless applications include private land mobile radio (e.g., police, dispatch), cellular, PCS, satellite, wireless local loop, and others. Wireline applications include ADSL, high speed modems, and data storage. In such systems, information is converted to information symbols, typically binary in value, which are encoded and modulated to create a form that can be transferred via a transmission medium such as wires, the air (e.g., using radio waves), or magnetic tape. Typically, the symbol values are passed through pulse shaping filters prior to transmission so that the transmitted signal will have a compact power spectrum.
In wireless communications, radio spectrum is shared between multiple communication channels. A combination of frequency division multiple access (FDMA), time division multiple access (TDMA) and code division multiple access (CDMA) is typically used. Space division multiple access (SDMA), which allows for reuse of channels in spatially separated areas, is also known. The multiple access problem is often encountered in wireline and data storage applications as well. Thus, while the discussion below focuses on wireless communications, those skilled in the art will appreciate that analogous problems and solutions are also applicable in wireline and data storage systems.
Most wireless systems include an FDMA component, in which an available frequency spectrum is divided into multiple frequency bands, each corresponding to a different carrier frequency. When closely spaced, or adjacent carriers are used to transmit information simultaneously, interference between the respective carrier frequencies or radio channels arises, and communications quality can be diminished. Thus, an ability to operate in the presence of adjacent channel interference (ACI) is essential if high communications quality and customer satisfaction are to be achieved.
Further complicating the adjacent channel interference problem is the fact that, as demand for communications grows, ever greater spectral efficiency is required. In an FDMA system, such spectral efficiency is achieved through tighter carrier spacing which allows for more carriers to be used within a given spectrum allocation. This in turn requires further receiver resilience to adjacent channel interference.
In conventional radio receivers, bandpass filtering is used to separate FDMA channels, and each FDMA channel is processed and demodulated separately thereafter. However, because the filtering function is not perfect, adjacent channel interference is inevitably contained within the filtered signal. Traditionally, adjacent channel interference was ignored or treated as noise in the channel demodulation process. More recently, radio frequency (RF) processing techniques for compensating for adjacent channel interference have been proposed.
One such technique is described in S. Sampei and M. Yokoyama, "Rejection Method of Adjacent Channel Interference for Digital Land Mobile Communications," Trans. IECE, Vol. E 69, No. 5, pp. 578-580, May 1986, which is incorporated herein by reference. The cited method teaches that, during demodulation of a given carrier signal, a bandpass filter centered at an adjacent carrier is used to extract an adjacent channel signal (ACS) at the adjacent carrier. The extracted signal is then used to estimate the adjacent channel signal envelope and carrier and to coherently detect the adjacent channel signal. The detected adjacent channel signal is then waveform shaped, and the estimated adjacent channel carrier and envelope are impressed on the resulting signal. Ideally, the described process provides a reconstructed adjacent channel signal at its carrier frequency. The reconstructed signal can then be passed through a bandpass filter centered at the carrier of interest and subtracted from the received signal to remove the adjacent channel interference.
Such an approach has several limitations, however. For example, analog signal processing using filters and mixers adds undesirable cost and size to a radio receiver, and since the analog components vary with the manufacturing process, such receivers provide a relatively unpredictable range of performance. Additionally, subtracting a signal at radio frequency requires highly accurate carrier reconstruction and time alignment, as an error as small as half a cycle at radio frequency can cause the adjacent channel signal to double rather than diminish. Furthermore, such use of the adjacent channel carrier (phase and frequency) and envelope (amplitude) implicitly assumes that the radio channels are not dispersive. However, in many practical wireless systems (e.g., D-AMPS and GSM), the symbol rate is sufficiently high that the radio transmission medium must be modeled to include time dispersion which gives rise to signal echoes. Thus, the proposed technique is not always practical for use in many present day applications.
Accordingly, there is a need for improved methods and apparatus for enhancing receiver performance in the presence of adjacent channel interference.